The field of optical telecommunication comprises both long-haul and short-distance applications. Long-haul applications involve transmission over relatively long distances, relatively high bandwidths, and relatively little branching of signals into separate channels. For such applications, fiber optics has generally been regarded as the most suitable technology for transmission and distribution of signals because it offers low loss and high bandwidth. By contrast, short-distance applications, such as loop networks and local area networks, typically involve transmission over short distances and branching of signals into many separate channels. For such applications, the cost of optical components is generally a significant factor, whereas loss and bandwidth are typically less significant relative to long-haul transmission.
Furthermore, single-mode transmission is currently favored for long-haul transmission because of its low pulse dispersion, but this requirement is relaxed in the case of short-distance transmission because pulse dispersion can be small over short distances. As a consequence, multimode transmission is a viable alternative for short-distance applications. Because multimode transmission channels generally have larger physical dimensions than single-mode channels, and manufacturing tolerances are consequently not as fine, a greater variety of materials and manufacturing methods are available for making short-distance components.
It has been suggested that at least some of these materials and methods have the potential of providing relatively inexpensive components that can be used in a commercially viable way to make short-distance networks that include many passive optical components such as splitters and combiners. Examples of such networks are loop networks and local area networks, as noted. Further examples are multipoint connections within signal processing machines such as computers and digital switches. Within such machines, it is proposed to have high-speed optical communication busses interconnecting many transmitters and receivers. In this regard, optical networks have been proposed for interconnecting circuit boards or even higher-level structures (such networks are referred to as "optical backplanes"), as well as for interconnecting individual chips on a single printed wiring board (PWB). Particularly for optical backplane and PWB applications, it is desirable to provide waveguides in the form of thin films that can be mounted on planar substrates such as those currently used for mounting electronic components.
Optical backplanes based on optical fibers have been proposed. Such backplanes frequently use fused fiber couplers, for example star couplers, for distributing signals. However, although they are useful, fused fiber star couplers are expensive. For example, in 1988 the cost of a typical 1.times.8 coupler was about 800 dollars.
Another approach based on optical fibers uses mixing rods to form splitters and combiners. Mixing rods are described, for example, in U.S. Pat. No. 4,072,399, issued to R. E. Love on Feb. 7, 1978. One embodiment of a mixing rod is an elongated cylinder of a material having an index of refraction matched to the cores of the fibers to be coupled, and clad with lower index material. The rod has polished endfaces oriented perpendicular to the longitudinal axis. The ends of optical fiber bundles are disposed adjacent to the endfaces. This approach is relatively inexpensive, although cost savings are reduced by the complexity of the assembly process. Furthermore, this approach is not easy to miniaturize or adapt to a planar geometry.
Various approaches have been taken to the formation of planar waveguide circuits from glass or polymeric material by photolithographic processing. Such an approach is discussed, for example, in U.S. Pat. No. 4,878,727, issued to A. A. Boiarski, et al. on Nov. 7, 1989. In yet another approach, notches are machined at designated coupling points in polymer waveguides for reflecting light into or out of the waveguide. This approach is described, for example, in U.S. Pat. No. 4,733,093, issued to A. F. Graves and E. A. Munter on Mar. 28, 1988. Although useful, all of these approaches involve time-consuming processing steps that add cost to the final product.
The fabrication of single-mode integrated optical devices using embossing or molding techniques was proposed, for example, in G. D. Aumiller, et al., "Submicrometer Resolution Replication of Relief Patterns for Integrated Optics," J. Appl. Phys, Vol. 45, (1974) pp. 4557-4562. One method discussed in that work involved embossing grooves in PMMA substrates and subsequently filling the grooves with a photopolymer having a higher refractive index than the substrate. However, it appeared difficult to achieve the fine tolerances considered necessary to make a useful waveguide by such a technique.
Thus, practitioners in the field have until now sought, without success, a passive optical waveguide technology, capable of embodiment in automated or semi-automated manufacturing processes with low labor content and moderate capital expenses, making possible the production of low cost planar optical waveguides, and of articles that comprise such waveguides.